language/unsorted/flatten_test fails with dart2wasm-linux-chrome (but not with d8)

[osa1]

It looks like the runtime errors are not thrown synchronously, and with Chrome the test ends before throwing the exception.

I'm not sure if the test needs asyncStart/asyncEnd or this is a dart2wasm issue.

Minimal repro:

import 'dart:async';

class Derived<T> implements Future<T> {
  noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
}

test() async {
  // flatten(Derived<int>) = int
  Future<int> x = (() async => new Derived<int>())();
}

main() {
  test();
}

This fails with a runtime error in d8, but passes in Chrome.

[osa1]

Another example of a similar failure is the test language/nnbd/static_errors/unchecked_use_of_nullable_test. Minimal repro:

void main() async {
  int? x;
  x.noSuchMethod(Invocation.method(#toString, []));
}

Currently this test fails as expected in Chrome because it fails with a Wasm trap. When we fix null.noSuchMethod to throw "no such method" (done in https://dart-review.googlesource.com/c/sdk/+/307800) it starts to fail in Chrome because the exception is not propagated to the test driver and test succeeds unexpectedly.

[osa1]

One explanation why these tests pass with d8 but fail with Chrome can be that we currently use setTimeout for scheduling (this will change with https://dart-review.googlesource.com/c/sdk/+/305201), which in d8 does not wait before calling the callback. So some of the async callbacks will be called almost synchronously.

With https://dart-review.googlesource.com/c/sdk/+/305201 we override d8's setTimeout, but the new event loop implementation used in d8 waits for all tasks to complete, so these tests will still work as expected with d8 and fail with Chrome because the test driver we use in Chrome does not wait (and has no way to wait) for all events to complete.

[eernstg]

Note that the compile-time error in 'language/unsorted/flatten_test.dart' isn't relevant for the behavior of the flatten function, so we might want to delete line 36, or we could rewrite it to use a local function with a declared return type rather than a function literal (like the one in line 35).

Cf. dart-lang/language#3148, which is an investigation of the compile-time error. Cf. https://dart-review.googlesource.com/309280 which will adjust the test.

[eernstg]

Test landed (note that it was cleaned up quite extensively, but it still covers the same cases, and then some more). It was moved to $SDK/tests/language/async/divergent_flatten_test.dart.

[mkustermann]

The language/async/divergent_flatten_test and language/nnbd/static_errors/unchecked_use_of_nullable_test are passing now.

The tests should be using asyncStart/asyncEnd which I'll do in cl/368581

The issue with test runner not recognizing async tests has been fixed some time ago.